Gelled fracturing fluids



. GELLED FRACTURING FLUIDS Raymond W. Starmann, Evergreen Park, and PaulGibson, Chicago, 111., as'signors to Swift & Company, Chicago, 111., acorporation of Illinois No Drawing. Filed May 8, 1956,Ser. No. 583,367

12 Claims. (Cl. 2528.55)

This invention in general relates to hydraulic pressure treatment ofsubterranean gas and/ or oil bearing formations. More particularly, theinvention relates to means for providing gels of liquidhydrocarbons-said gels having improved consistency inviscosity-temperature relationships and adapted for use in hightemperature wells where viscosity build-up has heretofore been aproblem.

The process which has become known as hydraulic fracture, orhydrofracing as it is often called, of oil or gas bearing formations isachieved by exertion of increasing quantities of pressure on a liquid inthe well bore until pressure is built up sufliciently to produce What isthought to be a fracture of the formation. It is generally ac- ;ceptedthat fracture usually occurs along horizontal bedding planes where thetensile strength of the formation is usually the weakest, butoccasionally, and particularly in non-Stratified or unconsolidatedformations, fracture is thought to occur along a vertical plane. In theprocess, fluid is pumped into the well at a rate which causes thepressure to rise until there is a sharp break in pressure (in theory thepoint of fracture) after which the pressure remains fairly constant at alow level as the fluid in theory penetrates the new crack. Thereafter,the pressure is relieved, and the well is put into production.

It has been established that the release of hydraulic pressure after thesharp break in pressure does not result in return of the formation toits original condition, but rather, the formation exhibits a substantialincrease in permeability. However, it has been established that thepermeability of the formation may be further increased ,bythe use ofoil-inert small particles such as sand, metal chips, crushed shell, etc.These particles are suspended in the hydraulic fluid. Under the fracturetheory, they enter the formation with the fluid at the point offracture. When the pressure is released and the fracturing fluid flowsfrom the formation, these materials remain to prop the formation apartto some extent at the point of fracture.

The hydraulic fluids which are generally used are miscible with theinterstitial crude oil, e.g., oily liquids,-such as crude petroleumoils, a refined fraction of crude oils, e.g., gasoline, kerosene, dieseloil, naptha, etc., and, much less often than the above, animal,vegetable and fish triglyceride oils.

The viscosity of these fluids often is increased by the addition ofother compositions to'the base fluid. In the case of petroleum oils orits fractions, gelling agents, which tend to thicken the oil, enable thesuspension of the small particles in the oil. Soaps of fatty acids suchas alkali metal soaps of red oil and tall oil fatty acids commonly areused as gelling agents in petroleum base fluids. In addition to alkalimetal soaps, other 'fatty acid soaps of metals of the alkali metal andalkaline earth series, or polyvalent metals such as cadmium, mercury,cobalt, lead, nickel and aluminum have been suggested as gelling agentsfor petroleum base fracturing fluids. Recently, the use as gellingagents for liquid hydrocarbons of alkali metal soaps of marine oil fattyacids (such as menhaden oil, herring oil, whale oil, seal oil, tuna oil,etc.), preferably fully hydrogenated, and mixtures thereof with otherhigher fatty acids in ratios such that the C and above acid content isat least 10%, has been developed. This subject matter is disclosed andclaimed in a copending application of Charles S. Steiner and PaulGibson, Serial No. 583,352, filed concurrently herewith, for aninvention entitled Liquid Hydrocarbon Gels and Uses Thereof."

We are aware that the theory of hydraulic fracture is not universallyaccepted. There are those who propose that the fluid jets issuing fromperforations in the casing at the pay zone etch cuts through the blockedarea immediately surrounding the bore hole to the more permeable areasmore remote from the bore. Others have proposed that the hydraulic fluiddrives the water block immediately surrounding the hole into the moreremote areas of the formation Without actual fracture. Regardless oftheory, it has been observed that the application of increasing pressureto a hydraulic fluid in the well bore results in a sharp break inpressure, after which the pressure levels off at a substantiallyconstant lower level. This type of treatment of pay zones in generalresults in increased production of oil and/or gas. In View of the factthat the term hydraulic fracture has been widely accepted by those inthe art to describe the foregoing process of well treatment, we shallemploy the same terminology to describe the process of hydraulicpressure treatment with the understanding that We are not limitedspecifically to any particular theory.

The process of the instant invention comprises the addition of aninorganic ammonium salt to a gelled liquid hydrocarbon, the gellingagent comprising an alkali metal soap of a higher aliphaticmonocarboxylic acid having between 14 and 24 carbons. Tall oil fattyacids, which contain some abietic acid, are considered to be a low gradeof fatty acids for purposes of this invention.

We have noted that with the aforementioned gelling agents, particularlywith the saturated acids, high gel viscosities develop at temperaturesin the range of 220 to 250 F. as what appears to be a secondary gelformation occurs. When wells having high bottom hole temperatures arefractured with these gelled fluids, the high temperatures cause the gelsto thicken to the point where they become difiicult to remove. Thisthickening can occur either in the formation or as the fracture isbrought to the surface after completion of the fracture operation. Inaddition, the gels become markedly more fluid (less viscous) as thetemperature increases up to the point of viscosity build-up as secondarygel formation begins, and sand suspension properties thereby diminish.

We have discovered that the addition of an inorganic ammonium salt tothe gel improves the fluid characteristics of the gel with increasingtemperature. The characteristic loss in viscosity is substantiallylessened by the presence of the ammonium salt. In addition, the ammoniumsalt prevents the formation of the highly viscous secondary gel attemperatures in the range of 220-250 F.-'-a problem of seriousproportions particularly in gels made with soaps of saturated fattyacids.

It is important in the perforamnce of the process of this invention thatthe ammonium salt is added after the gel is formed. The salt may be ineither solid form or in concentrated aqueous solution. While we do notwish to be limited to any particular theory, it would appear that whenthe temperature of the gel is raised sufiiciently,

'base exchange occurs between the alkali metal and ammonium ions. Whenthis occurs, the gel breaks because the ammonium soap is not temperaturestable insofar as gel properties are concerned.

Thus, the invention in practical use involves the formation of thegelled petroleum oil, either crude or a refined fraction thereof, by theaddition of caustic to the oil containing between about 0.5-3.0% byweight of the aforementioned acids. Sand or other propping agents areadded either before or after the gelation. After the gel has formed, theammonium salt, preferably in aqueous solution, is added to the tank andmixed therewith. About 1-3 equivalents of ammonium salt is added perequivalent of caustic used. This is roughly 1.5-5 .0 parts by weightofammonium salt per part of caustic. In the final gelled composition,the ammonium salt is present in, about 1-3 equivalents per mol of alkalimetal, the latter being present as the cation in the soap and in excesscaustic. The fluidis then ready to be pumped into the well to exertpressure on the formation.

The ammonium salts are preferably salts of a mineral acid such asammonium sulfate, ammonium chloride, ammonium nitrate or ammoniumphosphate. However, other ammonium salts such as ammonium acetate,ammonium citrate, ammonium carbonate, ammonium oxalate, ammoniumbromide, etc., may be used if desired.

The decomposition temperature of the inorganic salt has an effect on thetemperature at which the gel breaks. With salts of strong acids, the gelbreaks between about 190240 F.; whereas with the less stable ammoniumcarbonate, destruction of the gel occurs at much lower temperatures.

The following examples illustrate gel compositions falling within thescope of the present invention, it being understood that such examplesare in nowise limitative of the scope of our invention. All parts are byweight.

Example I Kerosene 100.0 Gelling agent:

Red oil 2.0 Caustic 0.5 Ammonium sulfate 1.5

Example II Kerosene 100.0 Gelling agent:

Hydrogenated herring oil fatty acids 1.5 Caustic 0.5 Ammonium nitrate1.2

Example III Low asphaltic crude oil 100.0 Gelling agent:

Hydrogenated menhaden oil fatty acids 2.5 Caustic 1.5 Ammonium sulfate6.0

Example IV Naphtha 100.0 Gelling agent:

Tall oil fatty acids 3.0 Caustic 0.5 Ammonium acetate 2.5

Example V Gasoline 100.0 Gelling agent:

Hydrogenated menhaden oil fatty acids 2.5 Caustic 0.5 Ammonium phosphate2.0

Obviously, many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and there fore, only such limitations should beimposed as are indicated in the appended claims.

We claim:

1. In a method of hydraulic pressure treatment of subterraneanformations, the steps which comprise: mixing liquid hydrocarbon,aliphatic, monocarboxylic acids having 14-24 carbon atoms, and causticto form a gelled liquid hydrocarbon, adding to the gel an ammonium saltcapable of undergoing a base exchange with said caustic, and thereafterpumping said gelled fluid into the formation until there is a sharpbreak in pressure.

2. In a method of hydraulic pressure treatment of subterraneanformations having high temperatures, the steps which comprise: mixingliquid hydrocarbon, aliphatic, monocarboxylic acids having 14-24 carbonatoms, and caustic to form a gelled liquid hydrocarbon, adding to thegel 1-3 equivalents of an ammonium salt per equivalent of caustic, andthereafter pumping said gelled fluid into the formation until there is asharp break in pressure.

3. A gelled composition which comprises: a liquid hydrocarbon gelledwith a gelling agent comprising an alkali metal soap of a mixture ofaliphatic monocarboxylic acids having between 14 and 24 carbon atoms;1-3 equivalents of an ammonium salt of a mineral acid per mol of alkalimetal in said gel; and oil-inert small particles suspended therein.

4. A gelled composition which comprises: a liquid hydrocarbon gelledwith a gelling agent comprising an alkali metal soap of higher aliphaticmonocarboxylic acids; a solution of an inorganic ammonium saltcontaining about l-3 equivalents of ammonium per equivalent of alkalimetal; and oil-inert small particles suspended there- 5. The compositionof claim 4 wherein the ammonium salt is ammonium sulfate. 7

6. The composition of claim 4 wherein the ammonium salt is ammoniumnitrate.

7. The composition of claim 4 wherein the ammonium salt is ammoniumchloride.

8. The composition of claim 4 wherein the ammonium salt is ammoniumphosphate.

9. A gelled composition which comprises: a liquid hydrocarbon gelledwith 0.5-3.0% by weight of a gelling agent comprising an alkali metalsoap of an aliphatic monocarboxylic acid having between 14 and 24 carbonatoms, dispersed in said gel; an aqueous solution of an inorganicammonium salt in an amount to provide 1-3 equivalents of said salt permol of alkali metal in said gel; and oil-inert small particles suspendedtherein.

10. The composition of claim 9' wherein said ammonium salt is a memberof the group consisting of ammonium sulfate, ammonium nitrate, ammoniumchloride, and ammonium phosphate.

11. The composition of claim 9- wherein said aliphatic acids arehydrogenated marine oil fatty acids having substantial amounts of C andabove acids.

12. The composition of claim 11 wherein said ammonium salt is a memberselected from the group consisting of ammonium sulfate, ammoniumnitrate, ammonium chloride, and ammonium phosphate.

References Cited in the file of this patent UNITED STATES PATENTS2,595,557 Worth et al. May 6, 1952 2,596,844 Clark May 13, 19522,667,457 McChrystal Jan. 26, 1954 2,753,364 Boner et al. July 3, 19562,779,735 Brown et al. Jan. 29, 1957

3. A GELLED COMPOSITION WHICH COMPRISES: A LIQUID HYDROCARBON GELLED WITH A GELLING AGENT COMPRISING AN ALKALI METAL SOAP OF A MIXTURE OF ALIPHATIC MONOCABOXYLIC ACIDS HAVING BETWEEN 14 AND 24 CARBON ATOMS, 1-3 EQUIVALENTS OF AN AMMONIUM SALT OF A MINERAL ACID PER MOL OF ALKALI METAL IN SAID GEL, AND OIL-INERT SMALL PARTICLES SUSPENDED THEREIN. 